1. Field of the Invention
This invention relates to a method of microwave assisted chemical reactions, such as sample preparation, synthesis, derivatization or extraction which involves reduced pressure within the vessel and associated apparatus for accomplishing this objective.
2. Description of the Prior Art
The use of microwave energy in analytical chemistry to provide heat to assist chemical reactions has been known for approximately 15 years. See, generally, Mingos et al., Applications of Microwave Dielectric Heating Effects to Synthetic Problems in Chemistry, Chem. Soc. Rev. 1991, 20, pp. 1-47.
It has been known to employ such microwave heating in sample preparation. See, Kingston et al., Comparison of Microwave Versus Conventional Dissolution of Environmental Applications, Spectroscopy 7 (9) November/December 1992, pp. 20-27. One approach involves an open-vessel approach in which the result is achieved with the assist of microwave heating. An alternate approach is the so called "closed-vessel" microwave sample preparation.
It has been know to use microwave energy for various types of environmental processes. For example, microwave energy, such as that produced by a nominal or high intensity microwave oven, has been employed to extract pesticides from sediment samples. See, Onuska et al., Extraction of Pesticides from Sediments Using a Microwave Technique, Chromatographia, Vol. 36, pp. 191-194 (1993). Microwave heating has also been employed in effecting hydrolysis of proteins. See, Margolis et al., The Hydrolysis of Proteins by Microwave Energy, Journal of Automatic Chemistry, Vol. 13, No. 3, pp. 93-95 (May/June 1991).
It has also been known to employ microwave energy in a closed vessel digestion system wherein a closed Teflon PFA vessel has an organic sample, an inorganic sample or a combination subjected to acid decomposition under the influence of microwave energy. See, Kingston et al., Microwave Energy for Acid Decomposition at Elevated Temperatures and Pressures Using Biological and Botanical Samples, Anal. Chem., 58, pp. 2534-2541, (October, 1986).
In such closed vessel microwave sample preparation techniques, typically, one or more materials which will become the sample are mixed or dissolved in a suitable liquid reagent. The liquid reagent occupies a portion of the volume of the relatively small vessel and is subjected to chemical alteration under the influence of the microwave heating, thereby creating a gas phase in addition to the liquid phase within the vessel. The microwave heating results in increased temperatures and pressures within the vessel which can present a potential safety hazard through vessel failure. The increased temperature is required for advancement of the reaction rate, but the pressure is a property of the heat flow characteristics of the vessel and microwave interaction.
It has been known to control heat loss from the vessel by providing a jacket of thermal insulation around the vessel which also acts to strengthen the vessel. See, generally, Mingos et al., Applications of Microwave Dielectric Heating Effects to Synthetic Problems in Chemistry, Chem. Soc. Rev., 1991, 20, pp. 1-47 and Chapter 6, Introduction to Microwave Sample Preparation Theory and Practice by Kingston et al., American Chemical Society, 1988, pp. 93-154.
U.S. Pat. No. 5,215,715 discloses a method of digesting materials which are dispersed in an acid digesting medium, which dispersion is subjected to microwave heating in a first chamber and then both the gas and liquid phases of the dispersion are cooled in another chamber. There is no segregated cooling of the gas phase while heating the liquid phase. There is also no recognition of the pressure relationship between the gas phase and liquid phase during microwave radiation.
In prior art practices, pressure within the vessel has been permitted to form at whatever natural level occurred due to the specific reagents, temperature, reaction products, microwave interaction and heat flow of the vessel.
There remains, therefore, a very real and substantial need for a more efficient and safe means of microwave sample preparation in a closed vessel.